Magnetic excitations in Sr₂IrO₄ measured with inelastic neutron scattering

The widespread current interest in 5d materials stemmed from the observation that relativistic spin-orbit coupling drives a Mott-like insulating ground state with pseudospin J_eff=1/2 magnetic moments in the iridate compound Sr₂IrO₄. One surprising and enduring aspect of the physics of Sr₂IrO₄ is the observation of similarities to the parent unconventional cuprate La₂CuO₄. The degree to which this analogy holds, and if this suggests a proximate unconventional superconducting regime in Sr₂IrO₄, stands as an important outstanding question. In particular, a central issue in Sr₂IrO₄ is to understand how the strong spin-orbit coupling affects the magnetic excitations. Inelastic neutron scattering (INS) allows direct access to the magnetic excitations, but measurements have so far been lacking due to several hurdles posed by iridates. Utilizing time-of-flight INS with a gram sized single crystal array we successfully overcame these challenges. The results reveal direct evidence for two-dimensional in-plane magnetic interactions in Sr₂IrO₄ and allows a definition of the spin-gap. Modelling of the INS results with an isotropic 2D Hamiltonian supports a robust analogy with the physics of unconventional cuprates.